Fuel cells electrochemically convert fuels and oxidants to electricity. Fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidant such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In a typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. Typically, gaskets seal these holes and cooperate with the longitudinal extents of the layers for completion of the fluid supply manifolds. As is known in the art, some of the fluid supply manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell.
The PEM can be made using, for instance, a polymer such as the material manufactured by E. I. Du Pont De Nemours Company and sold under the trademark NAFION.RTM.. Further, an active electrolyte such as sulfonic acid groups are included in this polymer. In addition, the PEM is available as a product manufactured by W. L. Gore & Associates (Elkton, Md.) and sold under the trademark GORE-SELECT.RTM.. Moreover, a catalyst such as platinum which facilitates chemical reactions is applied to each side of the PEM. This unit is commonly referred to as a membrane electrode assembly (hereinafter "MEA"). The MEA is available as a product manufactured by W. L. Gore & Associates and sold under the trade designation PRIMEA 5510-HS.
In a typical PEM-type fuel cell, the MEA is sandwiched between "anode" and "cathode" gas diffusion layers (hereinafter "GDLs") that can be formed from a resilient and conductive material such as carbon fabric. The anode and cathode GDLs serve as electrochemical conductors between catalyzed sites of the PEM and the fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) which flow in respective "anode" and "cathode" flow channels of respective flow field plates.
A given fluid flow plate can be formed from a conductive material such as graphite. Flow channels are typically formed on one or more faces of the fluid flow plate by machining. As is known in the art, a particular fluid flow plate may be a bipolar, monopolar, anode cooler, cathode cooler, or cooling plate.
The conversion of the fuel and oxidants to electricity in a fuel cell assembly also produces heat, particularly at high current/power densities, which is typically removed to maintain the fuel cell assembly at a safe operating temperature. For example, circulating liquid cooling systems have been employed to circulate cooling liquid through passageways in the fuel cell assembly to remove heat. Such circulating liquid cooling systems typically require a pump, connecting tubes, an expansion tank, radiator, thermal and/or other controls. In addition to the added expense and the complexity of integrating the circulating liquid cooling system with the fuel cell assembly, other drawbacks include the need to provide electrical power for operating the pump, the pump is subject to mechanical failure, the coolant may become contaminated or ionized resulting in electrical short circuits or shunts in the fuel cell assembly, and the coolant fluid may leak into the fuel cell reaction areas and/or freeze-up.
Other attempts to remove heat from fuel cell assemblies have incorporated heat pipes. For example, U.S. Pat. No. 5,262,249 to Beal et al. discloses a fuel cell employing a circulating liquid cooling system in combination with a heat pipe.
U.S. Pat. No. 5,064,732 to Meyer discloses a fuel cell having an anode and a cathode, an anode chamber, a cathode chamber, an electrolyte membrane, and a hydrophilic porous elements interposed between each cell stack. A regulator regulates the introduction of reactant gases directly into the cathode and anode chambers. The fuel cell stack temperature is regulated via a heat pipe system which removes waste heat. See also, U.S. Pat. No. 5,358,799 to Gardner, and U.S. Pat. No. 4,578,324 to Koehler et al.
A drawback with the above-noted fuel cell assemblies, which incorporate separate heat pipes, is that the heat pipes increase the length of the fuel cell assemblies which reduces the current/power density of the fuel cell assemblies. Another drawback with the above-noted systems is that providing separate heat pipes which are interspersed between the fuel cells, requires heat to be transferred across the interface between the fuel cell and the heat pipe.
Therefore, there exists a need for fuel cells and fuel cell assemblies which integrally incorporate heat pipes into the fuel cells so that the fuel cell assemblies may be compactly configured and heat may be readily and efficiently removed from the fuel cells. In addition, there is a need for fuel cell assemblies incorporating and employing heat pipes for transferring heat between the fuel cell and a supply of one or more reactant fluids for the fuel cells.